Friction

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What is friction?
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Whats Stopping You?
Fn
Mu
Friction
Hard to Live With It, Cant Live Without It
Coefficient of Friction
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Friction

• Friction causes objects to slow down.
• Friction creates heat.
• Friction degrades an objects energy

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Friction

• Opposes relative motion of two surfaces
• Consists of a matched pair of forces
• Obj1 pushes Obj2 while Obj2 pushes Obj1
• Equal magnitudes, opposite directions
• Comes in two types
• static and kinetic

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Places where friction is good

• The soles of shoes
• Car tires
• The feet of table legs
• Rock climbing holds
• Brakes
• etc

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Places where friction is bad

• In engines
• In transmissions
• On the bottoms of skis and snowboards (To a
  point)
• When you are trying to move a box by pushing it
  along the floor
• etc

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What causes friction?
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Source of Friction
At the microscopic level even the smoothest of
surfaces is dotted with little mountain peaks.
The tips of the peaks are the only parts that
touch the other material. Only a very small
portion of the apparent surface area is in
contact with the other surface This causes
extremely high pressures to form on the parts
that touch. This causes the two surfaces to
become welded almost at the points of contact

• Picture of extremely smooth mica with a scanned
  probe microscope

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Consider 2 Types of Friction
Force of Static Friction
This value represents the relative force
necessary to make an object move
Force of Kinetic Friction
This value represents the relative force
necessary to keep an object moving at a constant
rate
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Types of Friction

• Static Friction
• Acts to prevent objects from starting to slide
• Forces can vary from zero to an upper limit
• Kinetic friction
• -Sliding Friction
• Acts to stop objects that are already sliding
• -Rolling friction

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Friction is a Force That Resists Motion
N
F
fs
M1
Surfaces in Contact
Spring Scale
W
The Pink Block M1 Will not Move Until the Force F
(Pull on the scale ) Exceeds the Force of Static
Friction fs.
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Static Friction

• Increases linearly
• For a given pair of surfaces, the ratio of
  Frictional force to Applied force is a constant.

Frictional Force
Applied Force, N
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Max
Kinetic Region
Frictional Force Resisting Motion
Static Region
Force Causing the Object to Move
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Why?

• -The slightly larger value for static friction
  results from irregularities and contaminants on
  the surfaces and is less accurate in general than
  the coefficient of kinetic friction
• -inertia

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Coefficient of Static Friction
N
F
fs
M1
Surfaces in Contact
Spring Scale
W
Coefficient of Friction
Force Required to Cause Motion
Weight of Object
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Example of Static Friction

• What is the coefficient of static friction
  between a tabletop and a 2 kg block of wood if a
  2 N force is required to start the block moving?
• Identify knowns and unknown
• m 2 kg, applied force 2 N, v 0,
• µs ?

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• Appropriate equation f µ N.
• What is N?
• On a level surface the normal force upward is
  equal to the weight of the object downward, i.e.
  N W mg. So,
• f µ mg or µ f/mg 2 N/(2 kg 9.8m/s2)
• µ 0.102

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Static Kinetic Friction Coefficients
Material Coefficient of Static Friction ?S Coefficient of Kinetic Friction ?S
Rubber on Glass 2.0 2.0
Rubber on Concrete 1.0 0.8
Steel on Steel 0.74 0.57
Wood on Wood 0.25 0.5 0.2
Metal on Metal 0.15 0.06
Ice on Ice 0.1 0.03
Synovial Joints in Humans 0.01 0.003
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Application Analysis

• Sand is often placed on an icy road because the
  sand
• Decreases the coefficient of friction between the
  tires of a car and the road
• Increases the coefficient of friction between the
  tires of a car and the road
• Decrease the gravitational force on a car
• Increases the normal force of a car on the road

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Using the Gears-IDS Battery to Calculate The
Static
Coefficient of Friction
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Record the Maximum Force (F) (Before the
Battery Begins to Move)
Maximum Force F 110 g

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Record the Weight (W ) of the Battery
580 g
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The Coefficient of Static Friction Between the
Wood Desktop and the Plastic Battery is Described
Algebraically
110 g
580 g
.190
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The Coefficient of Kinetic Friction Can be Found
Using the Same Technique
Record the Force Required to Move the Battery at
a Constant Rate
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The frictional force depends only on- the type
of surfaces how hard the surfaces are pressed
together

• The frictional force depends only on
• 1-the type of surfaces
• 2-how hard the surfaces are pressed together

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What is the normal force?
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Block at Rest on a Table
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Normal Force

• From Newtons third law we know that if gravity
  or some other force pushes an object (like a
  block) into a second object (like a table) that
  second object will be exerting an equal force
  back on the first.
• Normal force is the force the table exerts back
  on the block
• Normal force is always exerted perpendicular to
  the surface
• Friction Force is always parallel to the surface
• So if the table is horizontal and gravity is the
  only force on the block Fn -Fg

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Normal force on a hill

• Normal force is exerted perpendicular to the
  surface in accordance with Newtons Third law
• No unbalanced force so the block is stationary or
  at least not accelerating

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• .

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Questions?
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Solution
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Frictional force and normal force.

• Frictional force is proportional to the normal
  force, f a N. On a level surface N W, but what
  if someone is lifting up on the object?
• Wont that reduce the normal force?

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Free Body Diagram
Lift from person,T
Normal,N

• N T W, so
• N W T.
• If T pulls at some angle, then just decompose
  into components.

Weight, W
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• Example
• A person pulls on a 300 N crate with a rope that
  makes a 300 angle to the ground. If the
  coefficient of static friction is 0.6, how much
  tension must the person exert to get the crate
  moving?
• Knowns W 300 N, ? 30o, µ 0.6.
• Unknown T ?
• Equations f µN. All forces balance when at
  rest.

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• Horizontal Tension T cos ?
• Vertical Tension T sin ?
• Net vertical force N Tsin ? W 0, so
• N W - Tsin ?.
• Net horizontal force Tcos ? f 0
• Tcos ? f µN µ(W - Tsin ?) , so

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• Tcos ? µ(W - Tsin ?)
• Tcos ? µ Tsin ? µW
• T(cos ? µ sin ?) µW
• T µW/(cos ? µ sin ?)
• T 0.6300N/(cos300 .6sin300)
• T 180N/(0.866.3)
• T 180 N/(1.166)
• T 154 N

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Experiment Does the friction force depend on
surface area?

• We stick a 1kg mass on a piece of on top of the
  clean surface and, using a Newton meter, pull the
  weight across the table with the string provided
  at constant speed.
• We put the same weight on a different piece of
  plexiglass with drastically different surface
  area and repeat
• What do you notice?

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What is rolling friction?
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• Rolling friction
• -nature
• -normal force
• -diameter

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• Rotational friction

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